The invention relates to the metal plating of plastics and, in particular, to enhancing the adhesion of metal plating to the plastic substrate of electronic circuit boards by treating the board with an alkaline permanganate solution prior to electroless metal plating of the board.
The metal plating of plastic parts is well-known to be of considerable commercial importance because the desirable characteristics of both the plastic and the metal are combined to offer the technical and aesthetic advantages of each. Thus, a part plated with a bright, metallic finish takes advantage of the economies in cost and weight afforded by substituting molded plastic parts for metal and, additionally, the plated finishes are not as susceptible to pitting and corrosion because there is no galvanic reaction between a plastic substrate and a plated metal.
An important process is the preparation of electronic circuit boards which requires the elctroless plating of a conductive metal layer, usually copper, onto the plastic substrate of the board, such as epoxy, and for convenience the following description will relate specifically to this process.
These boards vary in design and may have a copper layer on each surface of the epoxy (two-sided boards) or they can be multi-layer boards which have a plurality of inter-leaved parallel planar copper and epoxy layers. In both type boards throughholes are drilled in the board and metal plated to facilitate connection between the circuits on the copper layers. The throughholes present an additional plating problem because resin smear on the exposed copper caused by the drilling operation acts as an insulator between the metal of the through-hole and copper layer and must be removed prior to plating. The smear is usually removed using acid and this process degrades the physical integrity of the hole making it difficult to metallize and provides little or no adhesive support for the metallic deposit.
The problems in plating either the through-holes or other plastic parts of the board are well-known in the art and a number of methods have been developed to improve the adhesion of the metal plating to the epoxy. These methods generally employ oxidants to etch the surface of the plastic prior to plating and include chromic acid, sulfuric acid and acidic and alkaline permanganate solutions. The toxicity of the chromium compounds and their potential hazards as water pollutants and the safety precautions needed with sulfuric acid have increased the commercial use of permanganate solutions, particularly alkaline permanganate solutions, and a number of patents have been granted in this area.
U.S. Pat. No. 3,652,351 shows the etching of acrylonitrile-butadiene-styrene interpolymers (ABS plastics) using a composition containing manganate and hydroxyl ions. U.S. Pat. Nos. 4,042,729 and 4,054,693 disclose stable, highly active etching solutions containing particular ratios of manganate ions and permanganate ions by controlling the pH in the range of 11 to 13. U.S. Pat. No. 4,425,380 is specifically directed to cleaning through-holes of residual manganese prior to plating by contacting the etched plastic with a water soluble compound oxidizable by permanganate to reduce the manganese residues to a low oxidation state, e.g., SnCl.sub.2 -HCl, formaldehyde, followed by contacting with hot alkaline hydroxide. The disclosures of the above patents are hereby incorporated by reference.
The present invention relates to treating the plastic surface before plating and particularly to etching the plastic with an alkaline permanganate solution to remove smear and to improve the adhesiveness of the metal plate to the plastic board. U.S. Pat. No. 4,042,729, supra, attempted to improve the use of alkaline permanganate solutions to treat plastic circuit boards and notes the many problems associated with these solutions. Thus, acidic permanganate solutions are notoriously unstable, have a short life and rapidly decompose to manganese dioxide. Alkaline permanganate solutions present similar problems but, according to the invention, are highly active, easily controllable and stable provided the molar ratio of manganate ion to permanganate ion is up to about 1.2 and the pH of the solution is controlled in the range of 11 to 13 by the use of buffers or pH adjustors. The patent also provides a means for rejuvenating the bath to remove organics and proudce permanganate by disproportionation of manganate comprising bubbling CO.sub.2 into the bath to lower the pH from 12.5 to 11 to 11.5, heating to form permanganate ions and manganese dioxide from the manganate ions, cooling, precipitating carbonates and filtering to remove the manganese dioxide and carbonates. The preparation and control of the compositions and rejuvenating procedures of this patent are unwieldly and time consuming and there is a need for improved alkaline permanganate etching solutions and methods for using the solutions to etch plastic substrates.